As mobile communications apparatuses, such as mobile telephones, have small sizes, high-frequency devices used in the apparatuses are also demanded for having reduced dimensions. It is known that internal circuits in mobile communications apparatuses may be broken down by static electricity entering from the antenna terminal. The static electricity may be high, for example, thousands kilovolts per nanosecond.
FIG. 31 illustrates a conventional high-frequency device disclosed in Japanese Patent Laid-Open Publication No. 2001-127663. The conventional high-frequency device includes a high-pass filter (HPF) 1003 including a capacitor and an inductor and connected between an antenna terminal 1001 and a switching circuit 1002 for protecting the switching circuit 1002.
In order to have a large attenuation of signals of frequencies other than a passing band, the HPF 1003 requires to include more numbers of capacitors and inductors connected in multiple stages. The HPF 1003 including many capacitors and inductors has an insertion loss and may cause the high-frequency device to have a large size. Accordingly, as the size of the device is limited, the conventional high-frequency device can hardly have sufficient characteristics.
When a high-voltage noise, e.g., static electricity, having a frequency close to the passing band enters the conventional high-frequency device, the electricity is transferred from the antenna terminal 1001 via the HPF 1003 to the circuit 1002, thus breaking the circuit 1002.